Commonly assigned U.S. Pat. No. 4,786,765 and pending application Ser. No. 07/235,588, filed Aug. 24, 1988, disclose devices wherein the position of an implement, e.g., a pen, relative to a position detecting tablet is determined. Each stylus includes a tuned circuit having a resonant frequency approximately equal to the frequency of a wave derived from a coil arrangement of the tablet. The resonant frequency of the tuned circuit is varied slightly in response to differing characteristics associated with the pen. For example, the pen can be associated with differing colors, each having a different tuned circuit resonant frequency assigned to it. Alternatively, the pen includes a pressure transducer for varying the value of a reactance included in the pen as a function of pressure exerted by a stem of the pen on a surface of the tablet. The wave derived from the tablet coil arrangement is coupled to the tuned circuit, thence back to the tablet. The tablet responds to the wave coupled back to it to indicate the characteristic associated with the resonant frequency of the tuned circuit by comparing the phase angle of the wave supplied by the coil arrangement to the tuned circuit with the phase angle of the wave coupled back to the coil arrangement by the tuned circuit. In addition, the position of the implement is detected by coupling energy from the tablet to the implement. In the copending application, the same coil arrangement which enables the characteristic to be determined is used to detect implement position.
The tuned circuit of the prior art devices includes an inductor and capacitor series connected to each other, without any connection to a power supply. It is indicated in the '765 patent that the pressure responsive reactance can be a variable capacitor or a variable inductor. In the particular disclosed embodiment of the '765 patent, the variable inductance includes an elongated core or stem having a ferrite chip positioned therein so that the stem and chip are axially displaced and restored in response to a tip of the stem being urged against the tablet surface. Thereby, the resonant frequency of the tuned circuit is changed. The change in resonant frequency is detected as a change in phase angle.
A problem with the above-described structure is that as the ferrite chip is displaced with respect to the position of a coil which surrounds it, undesirable strokes are inevitably generated. The undesirable strokes occur because the stem must be translated on the order of 1 to 2 millimeters to achieve significant inductance change. The 1 to 2 millimeter displacement is significantly greater than the displacement of a conventional writing implement, such as a ballpoint pen or a felt tipped pen, during normal operation. Hence, a person using the prior art pen does not get the same feel that he has when using a conventional implement. It has also been observed that such displacement of the stem causes erroneous position indicating readings from tablets of the type disclosed in the co-pending application. In addition, the dynamic range of the inductance change is relatively low so that there are relatively small changes in the tuned circuit resonant frequency and therefore relatively small changes in the detected phase, as a function of stem displacement. Hence, the detection resolution of this prior art arrangement is relatively limited.
If the tuned circuit includes a conventional variable capacitor responsive to stem translation, a large force must be imparted to the stem relative to the normal pressure of a writing implement on a writing surface to provide a large capacitance dynamic range. The significant forces do not lend themselves to facile use of the implements when bearing against a surface of a tablet.
It is, accordingly, an object of the present invention to provide a new and improved variable capacitor having a large dynamic range in response to relatively small pressures and/or displacements being supplied to it.
A further object of the invention is to provide a new and improved variable capacitor having a large dynamic range, covering in excess of an order of magnitude, in response to a displacement of tens of micrometers of a mechanical element of the capacitor.
Another object of the invention is to provide a new and improved implement designed for use with a position detecting tablet, wherein the implement includes a variable capacitor having a large dynamic range associated with movement of a stem of the implement resembling the movement of the core of a writing implement.